1. Field of the Invention
The present invention relates generally to surface patterns for round objects. In particular, the present invention relates to surface patterns for golf balls, the surface patterns including both depressions and protrusions.
2. Description of the Related Art
Soon after the introduction of the smooth surfaced gutta percha golf ball in the mid nineteenth century, players observed that the balls traveled further as they got older and more gouged up. The players then began to roughen the surface of new golf balls with a hammer to increase flight distance. The bramble ball, which was introduced around the turn of the 20th century, was formed with bumps on the surface of the ball. Eventually, manufacturers began to manufacture golf balls having dimples formed in the outer surface.
The dimples on a golf ball are important in reducing the aerodynamic forces generated by a ball in flight as a result of the ball's velocity and spin. These forces, which overcome the force of gravity, are lift and drag.
The lift force acts perpendicular to the direction of flight and is a result of air velocity differences above and below the rotating ball. Recognition of this phenomenon is attributed to Magnus and is described by Bernoulli's Equation. Bernoulli's Equation, which is a simplification of the first law of thermodynamics, relates pressure and velocity:
            p      +                        1          2                ⁢        ρ        ⁢                                  ⁢                  V          2                    +              ρ        ⁢                                  ⁢        gh              =    c    ,where p is the pressure, ρ is the density, V is the velocity, g is the gravitational acceleration, h is elevation, and c is a constant along a streamline. We see from Bernoulli's Equation that pressure is inversely proportional to the square of velocity. With respect to the flight of a golf ball, the velocity differential—faster moving air on top of the ball and slower moving air on the bottom of the ball—results in lower air pressure on top and an upward directed force on the ball.
The drag force acts opposite to the direction of flight and orthogonal to the lift force. The drag force on a golf ball is attributed to parasitic drag forces, which consist of form or pressure drag and viscous or skin friction drag. A sphere is a bluff body, an inefficient aerodynamic shape. Therefore, the accelerating flow field around the ball causes a large pressure differential with high-pressure forward of the ball and low-pressure rearward of the ball. This pressure differential causes the flow to separate from the outer surface of the ball, resulting in the majority of the drag force on the ball. In order to minimize pressure drag, dimples are provided as a means to energize the flow field and delay the separation of flow, thus reducing the low-pressure region behind the ball. However, this reduction of pressure drag increases skin friction, which is due directly to the shear stress on the ball. Skin friction is a viscous effect residing close to the surface of the ball within the boundary layer, a thin layer of fluid (air) near the ball surface in which the velocity changes from zero at the ball surface to the free stream value away from the ball surface. The dimples provide an optimal amount of disturbance to trigger a laminar to turbulent flow transition while maintaining a sufficiently thin boundary layer region for viscous drag to occur.
One way to characterize the nature of a fluid flow is by its Reynolds Number. Reynolds Number, which is a dimensionless parameter, is a measure of the ratio of the inertia forces and viscous forces on an element of fluid within the flow:
      Re    =                  ρ        ⁢                                  ⁢        Vl            μ        ,where Re is the Reynolds Number, ρ is the density, V is the velocity, l is a characteristic length, and μ is the viscosity. At high Reynolds Numbers, the most effective method of reducing aerodynamic drag is to minimize the pressure drag. While present golf ball designs predominantly utilize dimples to trigger turbulence, early golf balls used protrusions to reduce drag, and some known golf balls have used a combination of concentric protrusions and depressions (dimples) to do so.
Some brambled golf ball designs have been presented in more recent times. For example, U.S. Pat. Nos. 4,836,552 and 4,839,116 disclose a short distance golf ball that may have a brambled surface in order to increase drag and thereby reduce velocity. The golf ball is designed for use on short golf courses.
U.S. Pat. No. 5,916,044 discloses a golf ball having dimples and protrusions, with each protrusion having a dimple positioned therein. This design allows a golf ball having a small nominal diameter to pass the USGA minimum diameter test.
U.S. Pat. Nos. 6,471,605 and 6,383,092 disclose a golf ball having a surface containing pyramidal projections. These projections are also used to allow a ball having a small nominal diameter to pass the USGA minimum diameter test.
What is needed is an improved surface pattern for golf balls that comprises both dimples and protrusions.